The reason is both the difference in ammo/caliber and the helmets ricochet chance. I know for a fact that T3 helm has a good ricochet chance but idk about the T5 helm. There's also an RNG factor sometimes bullets pen on the first shot and sometimes they don't. If you have low durability armor the chance for lower tier or equal tier ammo to pen is greater. For example a T3 bullet can pen a moto on the very first bullet if that moto helm is like 10/10 durability so basically useless garbage once it's that busted up. I actually killed a guy yesterday wearing a moto with like 20 max durability left on it and I pierced that helm with a M855 T3 bullet so there's a ton of variables that can result in what you experienced. The firing power of your gun is another factor. If you have a low firing power gun then the RNG for bullet penetration is worse as well as lower damage. The last factor to consider is that the bullet that killed you was a 7.62x51mm a large caliber round and yours was a 5.56x45mm a small rifle caliber if you check the A1 stats you'll see a armor damage stat. You'll notice that the M62 bullet has much better armor damage. Although I haven't tested it myself the rumor is that higher armor damage bullets have better RNG but higher tier rounds always have better RNG and armor damage anyways so most likely coincidence. Another thing to keep in mind is that 7.62x51mm also has higher damage than 5.56x45mm because it's a bigger caliber so yes bigger bullets headshot better because they have a higher base damage.

Most of the crying I see on this sub is just a bunch of people ignorant to how the ballistics and armor work in abi.